1. Field of the Invention
The present invention generally relates to bandpass filters, and more particularly to dielectric resonator filters which realize the most general transfer functions of bandpass filters.
2. Description of the Prior Art
High quality bandpass filters of narrow band width are required in many applications, including satellite transponder input multiplexers. Implementation of such filters in the past has been accomplished by using waveguide cavities in order to achieve the required large, unloaded Q. Considerable work has been done by the present inventor and others towards the realization of the most general bandpass transfer functions, including the elliptic function response and the more general transfer functions with finite complex transmission zeros, in waveguide form. U.S. Pat. Nos. 3,697,898 to Blanchier and Champeau; and 3,969,692 and 4,060,779 issued to the present applicant and Williams show how to construct relatively compact structures in waveguide form which realize the above-mentioned transfer functions.
Although the dual mode waveguide realizations of the most general transfer functions of bandpass filters represent a significant reduction of the weight and size of the multiplexers of communication satellites, the size and weight of these devices still represent a very significant portion of the payload. An example of such a dual mode filter is disclosed in the latter of the above-mentioned patents. The trend towards integration of various transponder components into microwave integrated circuit form (MIC) is not advanced with the use of waveguide multiplexers. Additionally, waveguide filters are relatively expensive components to manufacture, since the construction thereof involves extremely accurate machining operations with very tight tolerences.
The present invention takes advantage of recent advances in the developments of low loss, high relative dielectric constant materials, for example, ceramic barium titanate BA.sub.2 TI.sub.9 O.sub.20. Although bandpass filters of the all pole type (e.g. Tchebycheff, Butterworth, etc.) have been described using dielectric resonators, the most general transfer functions have not been previously realized in this form. It is the purpose of the present invention to illustrate how the most general class of bandpass filter functions, including elliptic functions and transfer functions with finite real, imaginary or complex transmission zeros, can be realized using dielectric resonators in a microstrip transmission line configuration. Since the dielectrics have a high relative dielectric constant, the size of a filter using the dielectric resonators is significantly smaller than a corresponding wave-guide filter. Further, since the transmission line medium in which the filters are realized is in the form of a microstrip, microwave integration is feasible with significant advantages both in production cost and in satellite transponder construction.
Examples of microwave filters employing dielectric resonators may be found in U.S. Pat. Nos. 4,184,130; 4,180,787; 4,132,233; 4,142,164; 4,135,133; 4,124,830; 4,121,181; 4,060,779; 4,028,652; 3,973,226; 3,969,692; 3,840,828; 3,713,051 and 3,697,898.